Conventional polyurethanes intended for thermoplastic processing are generally synthesized from one or optionally more relatively high molecular weight diols, a diisocyanate and a glycol--generally butane diol--as chain extender. These products may be processed very satisfactorily by injection molding. In contrast to this, where processing is carried out in extruders and/or on melt roll calenders, homogeneous melts free from particles which have remained intact are rarely obtained, i.e. only when optimum conditions which have to be determined from case to case are strictly observed. Even minor fluctuations in metering during the production of the thermoplastic polyurethane and minor variations in the extruder or roll temperature during processing are sufficient to lead to a melt heavily permeated by gel particles which, even after cooling, gives products having a rough surface. These gel particles have an adverse effect above all in the case of films. Not only do they have a different refractive index, but they also lead to stresses and, in some cases, to premature cracks in the event of elongation.
Thermoplastic polyurethanes have now been found which, in their homogeneous state, show virtually the same favorable properties as conventional products, but can be processed much more easily by extrusion and/or calendering to form homogeneous melts and then into hardened products having a smooth surface. These polyurethanes are characterized by the fact that a mixture of two low molecular weight diols is used as chain extender during their production. The expression "thermoplastic polyurethane" is abbreviated hereinafter to "TPU."
Although U.S. Pat. No. 3,214,411 describes, in Example 21, a TPU produced with a mixture of approximately 70% of 1,4-butane diol and approximately 30% of diethylene glycol as chain extender, the TPU thus produced has a considerably lower softening range and, on cooling from the melt, distinctly less favorable hardening behavior by comparison with a product produced with butane diol as sole chain extender. In addition, it shows a tendency towards pronounced shrinkage.